I got my CO2 tank from SWMBO's brother from his college days, some 3 or 4 years ago. There was some CO2 in there still and it worked just fine when I started using it.
IMO, no, it won't go stale. There are a lot of factors that can go into your beer's flavor changing over the course of three weeks.
"All other nations are drinking Ray Charles beer and we are drinking Barry Manilow."